Neisseria gonorrhoeae (Gc) is an obligate human pathogen that is the causative agent of the sexually transmitted disease gonorrhea. This gram-negative diplococcus is naturally found only within humans and has several interesting biological properties. Phylogenetic analysis of Gc indicate that it is distant from most other well studied gram-negative bacteria. All fresh human isolates of Gc express pili on their cell surfaces, and the expression of the pilus is required for infection in human volunteers. One unusual characteristic of Gc pili is the large number of possible pilus antigenic types that a single organism can produce (antigenic variation). This antigenic variation process occurs by DNA recombination between one of several silent pilin gene copies and the singular expressed gene (that encodes the major subunit of the pilus, pilin), resulting in multiple sequence changes in the expressed gene and protein. Pilin antigenic variation provides a large mosaic of antigenic types in a Gc population, and allows continual reinfection of the high-risk portion of the human population that transmits Gc into the general population. This antigenic variation system requires the RecA protein of Gc, as does DNA transformation and DNA repair, but it is largely unknown how RecA acts in these processes. When a Gc recA mutant is complemented with the E. coli (Ec) recA gene, the three RecA-dependent Gc processes are complemented differently: Ec RecA does not complement for DNA repair, but complements for DNA transformation, and hyper-complements for pilin variation. This proposal will define the molecular basis for these divergent complementation phenotypes, determine whether there are biochemical differences between Gc and Ec RecA, and determine if there are differences in protein-protein interactions that explain the alternate complementation patterns. These studies will aid in the understanding of gonococcal pathogenesis, and help define how the Gc and Ec RecA proteins mediate different cellular processes.